1. Field of the Invention
The invention concerns a radiological image detector of the type having a matrix arrangement of photosensitive elements. It concerns, more particularly, means to produce a light designed to achieve a resetting of the voltages at the terminals of the photosensitive elements.
2. Description of the Prior Art
The techniques used to make thin layer depositions, notably of amorphous silicon, are now properly mastered, and enable the making of large sized (40 cm.times.40 cm), high-resolution matrices of photosensitive elements which can, therefore, form digital type image detectors that are advantageously applicable to radiology.
In a standard way, photosensitive matrices comprise a network of row conductors and a network of column conductors. At each intersection of a row conductor and a column conductor, there is a photosensitive assembly called a photosensitive dot. The photosensitive dots are thus organized also in rows and columns. Each photosensitive dot is connected between a row conductor and a column conductor. In fact, to each row conductor, there are connected as many photosensitive dots as there are columns of these dots, and to each column conductor, there are connected as many photosensitive dots as there are rows of these dots.
Each photosensitive dot has at least one photosensitive element such as a photodiode, a phototransistor or a photoconductor, sensitive to visible light photons. These light photons are converted into electrical charge and this electrical charge is collected in an electrical capacitor forming a storage capacitor, constituted either by the capacitor of the photosensitive element itself or by an associated ancillary capacitor. A reading device enables the interrogation of the electrical state of the storage capacitor and the conveying of the electrical charge which forms the signal to a signal amplifier.
An example of a photosensitive matrix is given in a French patent No. 86 00656, published under number 2 579 319, which describes the detailed working of a photosensitive matrix as well as the appropriate reading method. Each photosensitive dot of this matrix is formed by a photodiode in series with a capacitor.
Another French patent No. 86 00716, published under the number 2 593 343, pertains to a matrix with a network of photosensitive dots, each formed by a photodiode and a capacitor in series, as mentioned above. This patent describes a method for the manufacture of a photosensitive matrix of this type, as well as a method for reading this matrix and an application of this matrix to the taking of radiological pictures. In order to pick up radiological images, the structure shown has a scintillator panel or screen which is subjected to an X-radiation. In response to this X-radiation, the scintillator screen emits radiation in visible light to which the photosensitive elements are sensitive.
The working of the photosensitive elements as described, for example, in the two above mentioned applications may require a stage for the restoring of the potential at the terminals of the photosensitive elements. This restoring of potential is called a voltage resetting stage (designated as RAN) in the above mentioned patent applications. This optic type of resetting stage can be achieved either by a calibrated uniform illumination, which may be of a given duration or may be permanent, and which produces a signal of little noise that is superimposed on the signal to be read, or by an intense light flash.
To achieve an optic type of resetting such as the one mentioned above, there is a known way to use a network of electroluminescent diodes, available in the market. Each of these electroluminescent diodes, in a standard way, forms an independent element, and these diodes are mounted on a panel so as to form a luminous panel giving a light-emitting surface with substantially the same dimensions as the matrix of photosensitive elements or photosensitive panel. The electroluminescent panel is applied against the photosensitive panel, and this set forms a digitized type of image detector.
A first drawback of this digitized image detector is that it is too thick to be interchangeable with a film cassette currently used in medical radiography instruments. This is due to the excessive thickness of the luminous panel. Another drawback lies in the complexity of the mechanical system for the support and assembly of the luminous panel/photosensitive panel set.